My 2005 Sierra'* transfer case (TC) has locked into 4WD Lo.

Dash selector switch light was at 4WD Lo location.
Replaced dash switch - no change.
Replaced fuse(*) - no change.
Tried shift change sequence (R-N-D-N-R) - no change.
Disconnected front drive axle and removed TC actuator.
Raised left wheel off ground.
Started truck and shifted transmission into neutral.
Tried to shift transfer case to 2H manually - TC spline won't shift.

Can anyone help?
Rob